Torque motors working as magnetic springs can be used for applications requiring rotary motion to develop torque with a limited angle of rotation. Magnetic springs accomplish this through attractive and repulsive electromagnetic torques of the rotor with respect to the stator. In some particular cases, an additional rotation of the stator is required, that is, both the rotor and the stator can rotate. The additional degree of mechanical freedom should not affect the level of the electromagnetic torque developed in both directions of rotation for completed revolutions.
Our U.S. Pat. No. 4,886,138 issued Dec. 12, 1989, to Graber et al. entitled "Electromagnetic Control Apparatus for Varying the Driver Steering Effort of a Hydraulic Power Steering System", is an application of one version of an actuator utilizing the principles of this invention. In particular, it has two degrees of freedom for conjoint rotation of the rotor and stator, as well as magnetically variable torque developed between the rotor and stator as a function of relative displacement and energizing current. The mechanism is used in conjunction with a torsion bar to center two portions of a power steering valve. By varying the current to the electromagnetic coil, the net centering force on the valve is varied and the steering effort is therefore controlled.
The magnetic springs can be used, of course, with a one degree of freedom configuration where the stator is stationary and only the rotor moves. Further, since the direction of torque is dependent on the direction of energizing current, the actuator can be used effectively to develop torque in either direction with respect to a center position. This leads to application as a three position actuator having stable states when centered or when driven to extreme positions on either side of center. The actuator can also be configured to seek an off-center position as a function of current where the actuator movement is restrained by an external spring force such that for a given current there is a specific position where the torque balances the spring force.
The electromagnetic mechanism of this invention includes a permanent magnetic circuit and an electromagnetic circuit. The permanent magnetic circuit comprises a pair of relatively rotatable elements, one of which is toothed to conduct magnetic flux and one of which includes permanent magnets for establishing a permanent magnet coupling. The electromagnetic circuit comprises an energizing coil and an external magnetic circuit and also shares the toothed element.
In one illustrated embodiment, the toothed element is defined by a pair of spaced magnetic pole pieces with a total of N circumferentially spaced interdigitated teeth to form an annular claw pole structure, and the permanent magnet element is defined by a ring element rotatably disposed inside the claw pole structure for flux coupling with the teeth. The ring magnet is supported for rotation with an output shaft and the pole pieces are supported for rotation with another shaft. The ring magnet has N alternating polarity radially magnetized regions (poles) about its outer circumference. Adjacent permanent magnet regions are magnetically coupled either internally or by an external flux conducting ring, and the interdigitated teeth are disposed in close proximity to such regions to define a working air gap therebetween. The electromagnetic circuit comprises at least one annular exciting coil disposed about the pole pieces and ferromagnetic pole elements positioned adjacent the magnetic pole pieces to couple flux from the exciting coil to the pole pieces.
The above elements define two magnetic flux paths: a permanent magnet flux path which includes (neglecting leakage flux) only the rotary magnet ring and the pole pieces, and an electromagnetic flux path which includes the coil, pole elements, the pole pieces and the magnet ring. When there is no energizing current in the exciting coil, the magnet ring moves to a stable position relative to the pole pieces, such that the center of each magnet region is aligned with a gap between the interdigitated teeth. Stated another way, the interfaces between magnet regions will be aligned with the centers of the pole piece teeth.
When the exciting coil is energized with direct current, the interdigitated teeth define N alternating polarity electromagnetic poles which interact with the poles of the permanent magnet. The flux path circles the coil, passing through the electromagnetic and permanent magnet poles and the pole piece. Each tooth assumes a polarity, depending on the direction of energizing current, which attracts a magnet region of opposite polarity and repels adjacent regions of like polarity, causing a torque from the center position toward a position where the center of each permanent magnet region is aligned with the center of a tooth. The direction of energizing current determines the direction of torque from the center position. The magnitude of the current and the displacement from the center position determine the amount of torque. This device has three stable positions and is ideally suited as a three position actuator. As will be further described, the device can be configured as a two position actuator or as an actuator which is positioned in accordance with the magnitude and direction of the energizing current.
In another embodiment, the pole pieces are stationary instead of being fixed to a rotatable shaft, thus offering one degree of freedom instead of two. The principle of operation is the same in either case.